Nearly any product or accessory you need as an aquarist can be purchased. Sometimes, however, that product isn't exactly what you want. This is a good thing for those do-it-yourselfers out there. I wouldn't want to begin calculating the amount of money I've spent in this hobby over the years, especially on items I wanted to customize that I couldn't. For this reason, I've spent lots of time learning how to make certain components. Youtube is a great place to find how-to videos for do-it-yourself projects. In fact, I've made a couple of internal filters following instructions I've found on Youtube, including the one in the photos above. I deviated from the actual how-to video quite a bit, but the concept is the same.
This is a simple internal filter made with a food-grade plastic squeeze bottle. The PVC tube is a spray bar for the pump outtake. For media, I used some bio plastic (lower two thirds of bottle) and pond sponge that I cut to fit (top third of bottle). Everything in the photos was purchased locally except the bottle itself, which I ordered online (a pack of 6). What you can't see is the hole in the bottom of the bottle where I attach it to the small water pump intake. The spray bar is then attached to the pump outtake. Water flows into the bottle through the holes cut into the lid. I've found this filter works great for quickly clearing up a tank that I've done significant work in or a tank I've wanted to clear of chemical treatment (medicine, etc.) I may have used for something. I put this in my tank and turn my primary filter off, which is great when your primary is a canister or something that would take extra time to unhook and swap out charcoal or other chemical filter media you might be using.
The Cichlid Room Companion
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